1. Field of the Invention
This invention relates to centrifugal pumps of the type employing mechanical seals.
2. Description of Prior Art
Centrifugal pumps are useful machines found in every day applications and especially in industry for moving many types of fluids, including those of a gel type or high particulate content. The centrifugal pump may have a casing defining an annular chamber with fluid inlet and outlet. An impeller rotated by a pump shaft is positioned in the chamber. Upon rotation of the impeller, fluid is moved through the chamber between the inlet and outlet. The pump shaft, as a rotatable member, is sealed in substantial fluid tightness to a wall member which closes one side of the chamber adjacent to the impeller.
In some situations, mechanical seals are used in centrifugal pumps when it becomes impractical to use conventional packing and sealing rings with radial sealing surfaces. In mechanical seals, the sealing surfaces are located in a plane perpendicular to the pump shaft. These sealing surfaces are highly polished and run upon each other with one surface carried by the shaft and the other surface on the pump's body. A spring urges these sealing surfaces into contact with only a thin film of liquid between them. Usually, the mechanical seal (single or double set) are mounted inside of a stuffing box to provide a usable seal assembly.
Mechanical seals will leak excessively if the pump shaft is laterally deflected so that the polished sealing surfaces are moved from the plane perpendicular to the pump shaft. In prior art pumps, the shaft deflection by bending was reduced by increased shaft strength and by placing the shaft bearings as close to the stuffing box as possible. However, many pump designs cannot be reconstructed to accommodate larger shafts or complex bearings, especially as retrofits on existing pumps having large volume production runs, or for other reasons.
Centrifugal pumps used in certain services require mechanical seals, and the fluid being handled has properties that create severe pump shaft bending moments. For example, the centrifugal pumps used for moving liquids loaded with solids such as rocks, sand or semisolids such as drilling mud (a non-newtonian fluid having thioxotropic properties) produce severe offcenter impeller loading on start-up of the pump. As a result, even shafts of one and one half inches in diameter can be flexed or bent adjacent the impeller of nearly one eighth of an inch in lateral deflection. Usually, a pump shaft deflection of greater than four to six thousandths of an inch will cause leakage or injury to a mechanical seal. An explanation of these severe bending problems of pump shafts resides in the settling of solids about the lower side of the impeller. Then, start-up of the pump produces severe flexing of the pump shaft before the impeller rotates sufficiently to clear its chamber of solids or solids like properties in drilling muds.
The present invention provides an improved centrifugal pump with a unique stuffing box assembly such that the remainder of the pump can be of conventional design. This stuffing box provides close mounting to the impeller of the mechanical seal and an antifriction (e.g. ball) bearing so as to reduce pump shaft flexing to a minimum amount tolerated by the mechanical seals. Further, the design of the stuffing box provides for a relatively simple pump structure to manufacture and easily employed for renewing the bearings or seals.